The present invention relates generally to ducts of the type currently employed for guiding electric conductors in switch or control boxes, that is, boxes or cabinets for accommodating switchgear and control gear.
Such ducts which are for example made of plastic typically have a U-shaped cross section and along the center line of their endwall uniformly spaced-apart openings for receiving fastening members necessary for securement to some sort of support member. These openings are usually longitudinally elongate slots for enabling the adjustment of the position of the duct relative to the support member on which it is to be mounted.
The present invention relates more particularly to the securement of such a duct on a support member which is previously installed in the switch or control box. Conventionally such a support member comprises a perforate plate generally made of metal. It therefore follows that in order to install the duct a control box must usually be equipped with such a perforate plate.
In addition to control boxes fitted with such perforate plates there are control boxes equipped with a frame comprising an array of support sections, generally made of metal, and of U-shaped cross section the legs or sidewalls having right angle bends or flanges for snapping on the desired switchgear or control gear.
According to standardized arrangements the legs may be of different heights and their right angle flanges are usually outwardly directed, away from each other and at the same level. As a variant such legs may be at different heights with respect to each other, and their right angle flanges are then usually inwardly directed toward each other, but at different levels.
In any event heretofore such ducts could not be easily mounted on such structural sections.
Moreover, the securement of ducts to a perforate plate has usually required the use of screws or even rivets. Such a fastening member must be picked out of a loose batch of such fastening members which is time-consuming and a source of losses of such fastening members.
When fastening members are threaded fasteners or screws, whether of metal or plastic, after passing through the endwall of a duct, the perforate plate must receive a nut from the remote side of the perforate plate. Since this requires the electrician or installer to work from the rear of the perforate plate it is in practice inconvenient and relatively time-consuming.
Further, as regards metal threaded fasteners, their use is contrary to safety regulations since they form a conductive path between the electric conductors in the duct and the subjacent perforate plate, to the detriment of normal unbroken insulation of the endwall of the duct.
As for the use of rivets, they require special tools whether they are made of metal or plastic.
In a copending application Ser. No. 565,764 filed on even date with the instant application, and assigned to assignee of the present application, there is disclosed a novel duct mounting fixture for the securement of a duct on a support section having sidewalls with right angle flanges. The duct mounting fixture comprises a base plate having an outer side which defines a bearing plane against which the endwall of the duct is adapted to bear. Means in the middle zone of the outer side of the base plate is adapted to receive a fastening member which is adapted to extend through the endwall of the duct via an opening therein and grip the endwall. At least two retaining teeth disposed on the underside of the base plate are adapted to cooperate with the right angle flanges of a support section, and at least one of the retaining teeth is carried by a resilient arm which is deformable in the course of engagement.
In practice, for such a duct mounting fixture to be used on either of two different types of support sections two pairs of separate retaining teeth are provided, each pair for one type of support section. The fastening member provided on the outer side of the base plate for gripping the endwall of the duct is in practice a rotatable latch member which turns 90.degree. after its head is extended through the endwall of the duct.